Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. In addition to electrocatalyst the electrodes may also comprise an electrically conductive substrate upon which the electrocatalyst is deposited. The electrocatalyst may be a metal black (namely, a substantially pure, unsupported, finely divided metal or metal powder) an alloy or a supported metal catalyst, for example, platinum on carbon particles.
A solid polymer fuel cell is a type of electrochemical fuel cell which employs a membrane electrode assembly ("MEA"). The MEA comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers.
A broad range of reactants can be used in electrochemical fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute oxygen stream such as air.
The electrochemical oxidation which occurs at the anode electrocatalyst of a solid polymer electrochemical fuel cell, results in the generation of cationic species, typically protons, and electrons. For an electrochemical fuel cell to utilize the ionic reaction products, the ions must be conducted from the reaction sites at which they are generated to the electrolyte. Accordingly, the electrocatalyst is typically located at the interface between each electrode and the adjacent electrolyte.
Effective electrocatalyst sites are accessible to the reactant, are electrically connected to the fuel cell current collectors, and are ionically connected to the fuel cell electrolyte. For example, if the fuel stream supplied to the anode is hydrogen, electrons and protons are generated at the anode electrocatalyst. The electrically conductive anode is connected to an external electric circuit which conducts an electric current from the anode to the cathode. The electrolyte is typically a proton conductor, and protons generated at the anode electrocatalyst migrate through the electrolyte to the cathode. Electrocatalyst sites which are ionically isolated from the electrolyte are not productively utilized if the protons do not have a means for being ionically transported to the electrolyte. Accordingly, coating the exterior surfaces of the electrocatalyst particles with ionically conductive coatings has been used to increase the utilization of electrocatalyst exterior surface area and increase fuel cell performance by providing improved ion conducting paths between the electrocatalyst surface sites and the electrolyte.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Increasing effective utilization of the electrocatalyst surface area enables the same amount of electrocatalyst to induce a higher rate of electrochemical conversion in a fuel cell resulting in improved performance.
Electrocatalyst materials used in electrochemical fuel cells typically comprise noble metals such as platinum. These materials are expensive, so in addition to improving performance, the present method may also be used to reduce the amount of noble metal used in an electrochemical fuel cell thereby reducing material costs.
U.S. Pat. Nos. 5,186,877 and 5,346,780, disclose methods of coating the exterior surfaces of electrocatalyst particles with a proton conductive film such as an ionomer to improve ionic conductivity between the electrocatalyst and the electrolyte. Known methods of applying exterior coatings to electrocatalyst particles include brush coating and using electrocatalyst/ionomer inks.
In conventional methods, the amount of ionomer coating on the electrocatalyst particles is controlled because too much ionomer can result in reduced electrocatalyst utilization due to poor electrical conductivity and/or reduced reactant accessibility to the electrocatalyst sites. Higher reactant accessibility is preferred to allow the reactants to access the electrocatalyst surfaces at a rate which will sustain the desired electrochemical reaction. As described above, too little ionomer can result in reduced proton conductivity and reduced electrocatalyst utilization.
A problem with conventional methods of coating electrocatalyst particles with ionomers is that they do not effectively impregnate the pores of electrocatalyst particles, especially where pores have aperture sizes less than 0.1 micron (hereinafter defined as micropores). Interior surface areas are defined as those surface areas which form the walls of cracks or pores in the electrocatalyst particles. When porous electrocatalyst particles which have been coated by conventional methods are used in electrochemical fuel cells, a significant portion of the electrocatalyst interior volume and the corresponding interior surface areas are not utilized.
Another problem with conventional methods is that the coatings applied to the electrocatalyst particles may actually obscure or block the pore openings. If micropore openings are blocked, reactants may be prevented from entering the micropores and accessing interior surface areas of the electrocatalyst particles, reducing electrocatalyst utilization and diminishing fuel cell performance.
Complete utilization of the electrocatalyst particle surfaces is limited in part by mass transport limitations within the pores of porous electrocatalyst particles. Thus there is a need for a method of improving the mass transport properties to enhance the reactant accessibility and ionic conductivity within the pores of the electrocatalyst particles.